This application takes priority from the Provisional U.S. Patent Application “Counter Balanced Transport for Blinds” filed on Mar. 23, 1999, Ser. No. 60/125,776, which is hereby incorporated by reference. The present invention relates to a modular transport system for opening and closing Venetian blinds, pleated shades, and other blinds and shades. While the embodiments shown herein are of horizontal blinds, the transport system may also be used on vertical blinds.
In order to proceed, it is necessary to explain the operation of a blind transport system and to define some of the terms used. Typically, a blind transport system will have a top head rail which both supports the blind and hides the mechanisms used to raise and lower or open and close the blind. The raising and lowering is done by a lift cord attached to the bottom rail (or bottom slat). Thus, when raising a blind, at first only the bottom rail is being raised and the amount of force required is small. As the bottom rail is raised further, more of the slats are stacked on top of the bottom rail and thus progressively more force is required to continue to raise the blind. The largest amount of force will be required at the very top when literally the entire blind is being raised. By the same token, the greatest amount of force will be required to keep the blinds in this fully raised position, as one is fighting against the weight of the entire blind.
In contrast, when the blind is fully lowered, only the bottom rail is supported by the lift cord. The rest of the weight of the blind is supported by the ladder tape which has tilt cables running to, and supported by, the head rail. Since the weight of all slats not resting on the bottom rail is supported by the head rail (via the ladder tapes), this weight need not be overcome when raising the blind. Only the weight of the bottom rail, and the weight of each successive slat as it comes in contact with the bottom rail as the blind is raised, need to be overcome.
In essence, the lift cord and the ladder tapes exchange loads as the blind is raised and lowered. The ladder tapes do practically all of the supporting when the blind is down. As the blind is raised, the weight is shifted from the ladder tapes onto the lift cords as each successive slat is picked up by the rising bottom rail and thus is no longer supported by the ladder tapes. The implication is that the least amount of force is required to start raising a fully lowered blind, and also the least amount of force is required to keep the blind in this lowered position. Progressively larger force is required to lift and to maintain the position of the blind as the blind is raised until a maximum amount of force is reached at the topmost position, where the blind is fully raised.
The force required to raise the blind varies directly and approximately linearly with the raising of the blind, increasing from a minimum when the blind is fully lowered to a maximum when the blind is fully raised. This same force also varies directly and approximately linearly with the size and weight of the window covering.
The basic concept for a blind transport system is described in U.S. Pat. No. 13,251, “Bixler”, issued Jul. 17, 1855, which is hereby incorporated by reference. However, the coiled spring motor used by Bixler is not a constant force motor. As the blind is pulled down, the spring is coiled tighter. Thus, the spring provides the strongest force when the blind is down, which is when the least force is required to assist in lifting the blind.
Other relevant blind transport systems provide a spring that gets stronger as the blind is lowered and weaker as the blind is raised, exactly the opposite of the desired effect. These systems may use a ratchet mechanism or brake to compensate for this shortcoming.
As the blind is lowered, its weight and the force of gravity are used to wind up the spring so that the unwinding of the spring may assist in the raising of the blind. In order to accomplish this raising of the blind, there is generally some type of mechanism to wind up the lift cord onto a shaft or spool. Preferably this mechanism will pull the lift cord vertically, with no horizontal component to upset the symmetry and functionality of the ladder tapes.
Many lift cord winding mechanisms have been used in the prior art. Typically they displace the wind-up spool axially as the lift cord is wound up, requiring a complicated mechanism, or they have problems with over wrapping and tangling of the cord. In order to prevent this over wrapping or tangling, some mechanisms guide the incoming coils of the lift cord axially along the spool using either a shoulder on the spool or a finger or kicker in close proximity to the surface of the spool. In the prior art, the kicker is located at the bottom of the spool, just before the point where the new lift cord enters. The weight of the blind pulls the spool downwardly, causing it to sag, and this can cause the gap between the kicker and the spool to be reduced to the point that there is interference between the spool and the kicker, creating friction.
As may be appreciated from the prior art, the purpose of the spring motors is primarily to assist in raising the blind. Thus, a mechanism must be found to transfer and control the force from the spring motor to the lift cords, and to do so such that all the cords are lifted the same amount simultaneously (so the blind is raised evenly), and such that the cords are pulled only vertically with no horizontal component.
A complete blind transport system must also include mechanisms to accomplish other tasks. Primary among these other tasks is the ability to open or close the blind via tilting of the individual slats. This is typically accomplished with ladder tapes (and/or tilt cables) which run along the front and back of the stack of blinds. The lift cords, in contrast to the tilt cables) typically run through slits in the middle of the slats and are only connected to the bottom rail.
When the blind is closed on a standard window shade, the slits through which the lift cords run become quite visible and allow light to pass through the blinds. It is desirable, for aesthetic reasons, to have a window covering product where there are no slits visible such that, when the blind is closed, there is no light passing through the blind. This is referred to as a “de-lighted” product and is a desirable product or feature.
The prior art shows that blind transport systems have traditionally been custom-designed and custom-built around the needs of a particular window covering. Each element in the transport system must be carefully fabricated and modified as required for it to meets its function as well as its physical placement within the system. All the different elements must be carefully mounted and placed so they will co-operate with each other and this is done at the expense of much time. Furthermore, changing even one single characteristic of the blind (such as going from lightweight vinyl to heavy wooden blinds, or simply increasing the width or the length of the window covering) necessitates going through the entire time consuming process of customizing the entire blind transport system. The nature of this process makes it expensive to truly customize a system in order to optimize its performance.